An electroluminescent device (EL device) is a self-light-emitting device which has advantages in that it provides a wider viewing angle, a greater contrast ratio, and a faster response time. The first organic EL device was developed by Eastman Kodak, by using small aromatic diamine molecules, and aluminum complexes as materials for forming a light-emitting layer [Appl. Phys. Lett. 51, 913, 1987].
The most important factor determining luminous efficiency in an organic EL device is light-emitting materials. Until now, fluorescent materials have been widely used as light-emitting material. However, in view of electroluminescent mechanisms, since phosphorescent materials theoretically enhance luminous efficiency by four (4) times compared to fluorescent materials, development of phosphorescent light-emitting materials are widely being researched. Iridium(III) complexes have been widely known as phosphorescent materials, including bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate) ((acac)Ir(btp)2), tris(2-phenylpyridine)iridium (Ir(ppy)3) and bis(4,6-difluorophenylpyridinato-N,C2)picolinate iridium (Firpic) as red, green and blue materials, respectively.
At present, 4,4′-N,N′-dicarbazol-biphenyl (CBP) is the most widely known phosphorescent host material. Recently, Pioneer (Japan) et al. developed a high performance organic EL device using bathocuproine (BCP) and aluminum(III)bis(2-methyl-8-quinolinate)(4-phenylphenolate) (BAlq) etc., as host materials, which were known as hole blocking layer materials.
Although these materials provide good light-emitting characteristics, they have the following disadvantages: (1) Due to their low glass transition temperature and poor thermal stability, their degradation may occur during a high-temperature deposition process in a vacuum, and the lifespan of the device decreases. (2) The power efficiency of an organic EL device is given by [(π/voltage)×current efficiency], and the power efficiency is inversely proportional to the voltage. Although an organic EL device comprising phosphorescent host materials provides higher current efficiency (cd/A) than one comprising fluorescent materials, a significantly high driving voltage is necessary. Thus, there is no merit in terms of power efficiency (lm/W). (3) Further, the operational lifespan of an organic EL device is short and luminous efficiency is still required to be improved.
Meanwhile, in order to enhance its efficiency and stability, an organic EL device has a structure of a multilayer comprising a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, and an electron injection layer. The selection of a compound comprised in the hole transport layer is known as a method for improving the characteristics of a device such as hole transport efficiency to the light-emitting layer, luminous efficiency, lifespan, etc.
In this regard, copper phthalocyanine (CuPc), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (MTDATA), etc. were used as a hole injection and transport material. However, an organic EL device using these materials is problematic in quantum efficiency and operational lifespan. It is because, when an organic EL device is driven under high current, thermal stress occurs between an anode and the hole injection layer. Thermal stress significantly reduces the operational lifespan of the device. Further, since the organic material used in the hole injection layer has very high hole mobility, the hole-electron charge balance may be broken and quantum yield (cd/A) may decrease.
Therefore, a hole transport layer for improving durability of an organic EL device still needs to be developed.
U.S. Pat. No. 8,227,798 B2 and Korean Patent Appln. Laying-Open No. 10-2010-0108924 disclose a compound wherein a nitrogen-containing heteroaryl such as triazine is bonded to a nitrogen atom of a dibenzocarbazole as an organic electroluminescent compound; Korean Patent No. 10-1074193 discloses a compound wherein a nitrogen-containing heteroaryl such as triazine is bonded to a nitrogen atom of a benzocarbazole as an organic electroluminescent compound; and Korean Patent Appln. Laying-Open No. 10-2014-0015259 discloses a compound wherein an aryl including anthracene is bonded to a nitrogen atom of a dibenzocarbazole as an organic electroluminescent compound. However, the above references do not specifically disclose an organic electroluminescent compound in which a quinazoline or quinoxaline is bonded to a nitrogen atom of a dibenzocarbazole directly or via a linker.